1. Field of the Invention
The present invention relates to a high pressure discharge lamp having an auxiliary lamp that emits ultraviolet rays for decreasing required voltage value for lighting-up. The auxiliary lamp enhances lighting-up performance of the discharge lamp.
2. Description of Related Art
High pressure discharge lamps are mainly used as light sources for optical devices, for example liquid-crystal projectors, exposure devices or the like. The high pressure discharge lamp comprises: an arc tube having a space where luminescent materials such as mercury, halide, products from halogen cycle, or the like are enclosed; and a pair of main electrodes placed oppositely each other inside the arc tube. An electrical discharge by an insulation breakdown between the main electrodes that high voltage for lighting-up is applied excites the luminescent materials. The excited luminescent materials emit light.
Recently, for reducing the size of point light sources and enhancing luminance efficiency of high pressure discharge lumps, an amount of the enclosed luminescent materials is increased and a volume of the space of the arc tube is decreased. Consequently, a pressure of the arc tube in operation is extremely high, for example approximately 200 atmospheres or higher. Furthermore, this kind of the optical devices is required to reduce the time not only for cold start but for hot start.
Particularly, the higher the pressure inside the arc tube is, the higher the required voltage for generating the electric discharge is. At the hot start, when inner temperature of the arc tube is high, it is required not only to apply high voltage but to wait until the temperature of the high pressure discharge lamp becomes relatively low. And high voltage, for example more than 10 kV, is required for the cold start.
But application of the high voltage in lighting-up of the high pressure discharge lamp causes a problem. For example, insulation breakdown at unintended position is generated and the insulation breakdown causes electrification. And noises generated by the application of the high pressure voltage causes malfunction of an electronic circuit provided for the optical device. The “insulation breakdown at unintended position” is, for example, insulation breakdown between two insulation cables or creeping discharge on a surface of a connector or a connecting terminal.
Technology for lighting-up the high pressure discharge lamp with lower voltage has been developed to solve these problems. For example, see Patent Document 1 (JP-A-2002-151006). A light source 1 of the Patent Document 1, as shown in FIG. 8, has a high pressure discharge lamp 2, an auxiliary lamp 3 separated from the high pressure discharge lamp 2, and a power feeding device 4. The high pressure discharge lamp 2 has an arc tube 5 and a feeder 6. The arc tube 5 has a light-emitting part 5a that has an inner space where mercury or the like, or luminescent material M1, is enclosed and a sealing portion 5b enclosing the inner space of the light-emitting part 5a. The feeder 6 has a pair of main electrodes 6a, a pair of metal foil 6b, and a pair of external lead bar 6c. Each main electrode 6a is provided to face each other within the light-emitting part 5a at a specified position. Each of the metal foils 6b is connected electrically to the corresponding main electrode 6a and buried in the sealing portion 5b. One end of each external lead bar 6c is connected electrically to the corresponding metal foil 6b and buried in the sealing portion 5b, and the other end of each external lead bar 6c protrudes outside.
The auxiliary lamp 3 has a discharge envelope 7, an inner electrode 8, and an outer electrode 9. The discharge envelope 7 has a discharge space enclosing medium for discharge M2 as a material that generates ultra violet UV by being excited with electrical discharge. And the discharge envelope 7 is provided toward an end face of the sealing portion 5b. The inner electrode 8 is provided inside the discharge space and connected electrically to one external lead bar 6c. One end of the outer electrode 9 is wound around an outer surface of the discharge envelope 7, and the other end of the outer electrode 9 is connected electrically to the other external lead bar 6c. 
In this light source 1, high-frequency voltage is applied between the inner electrode 8 and the outer electrode 9 of the auxiliary lamp. And electrical discharge is generated between the inner electrode 8 and the outer electrode 9 through the discharge space of the discharge envelope 7. The medium for discharge M2 in the discharge space excited with the electrical discharge generates ultra violet rays UV. When the ultra violet rays UV irradiates the main electrodes 6a in the light-emitting part 5a of the high pressure discharge lamp 2, the electrical discharge between the main electrodes 6a is accelerated. Consequently, the high pressure discharge lamp 2 can be lighted with lower applying voltage.